I have the same problem with a friend. I wasn't sure how to see whether someone was blocked or not using the Metabolt viewer, so I asked my friend to help. I blocked her, unblocked her, ok got it! But then when I logged on again, she was blocked.
I tried unblocking from the profile, sending an IM, sending a Linden, and all methods work until i log off and log back in.
I tried all of the above using Second Life viewer, Firestorm Viewer. I have used different computers, LINUX and still the automatic blocking can not be eliminated.
Ih short, it is either a problem at Linden (sounds like they may not care) or else it might be something somehow that the blocked person has inadvertantly done. I've given up trying to fix it though, and I just manually unblock my friend each time I log on.